In the manufacture of tapes, one of the used methods is to roll a sheet material in the nip of a calender and subsequently stretching the rolled sheet in one or more separate stretching stages, wherein the rolled sheet material is elongated in air between two sets of rollers, as for example disclosed in WO 2008/127562. During stretching the sheet material is reduced in width and in thickness. The reduction in width of a rolled sheet during stretching is called necking or neck-down. The reduction in width of the sheet material in air between the two sets of rollers is generally characterised by a very rapid neck-down, i.e. the width of the sheet material is reduced strongly over a very limited distance.
A rapid neck-down can result in differences in stretching behaviour over the width of the sheet material as the deformation rate of the sheet material can, for example, increase from the center to the outer edges of the sheet material. A rapid neck-down can also lead to differences in polymer orientation over the width of the stretched tape.
The differences in stretching behaviour over the width of the neck can cause thickness variations over the width of the resulting stretched tape. Furthermore, the stretched tape may exhibit stress differences when being strained, which in turn can cause the formation of unwanted wrinkles in the tape. These detrimental effects become more problematic with increasing width of the rolled sheet material.
The rolled sheet can also be stretched over one or more convex plates. The position of the start of the neck-down can vary in time in prior art processes resulting in varying mechanical properties in time and thus over the length of the manufactured tape.